[V0 deprecation] Remove no longer used get_metadata_cls (#28370)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>

vllm/v1/attention/backends/rocm_aiter_fa.py

@@ -9,7 +9,6 @@ import torch
 from vllm.attention.backends.abstract import (
     AttentionBackend,
     AttentionImpl,
-    AttentionMetadata,
     AttentionType,
     MultipleOf,
 )
@@ -479,10 +478,6 @@ class AiterFlashAttentionBackend(AttentionBackend):
     def get_impl_cls() -> type["AiterFlashAttentionImpl"]:
         return AiterFlashAttentionImpl
 
-    @staticmethod
-    def get_metadata_cls() -> type["AttentionMetadata"]:
-        return AiterFlashAttentionMetadata
-
     @staticmethod
     def get_builder_cls() -> type["AiterFlashAttentionMetadataBuilder"]:
         return AiterFlashAttentionMetadataBuilder